The goal of the proposed research is to identify the metal-ion binding sites of metalloenzymes which require Co(II) for activity and of enzymes in which Co(II) may be substituted for the divalent cation, such as Zn(II), normally required for activity. The proposed research will take advantage of the fact that, whereas Co(II) complexes ordinarily undergo substitution rapidly, Co(III) complexes undergo substitution very slowly. It is then expected that when enzyme-bound Co(II) is oxidized by a reagent such as hydrogen peroxide, the Co(III) complex will maintain bonds to the same amino acid residues to which Co(II) was bound long enough for the enzyme to be cleaved and the amino acid residues to be identified.